This invention relates to the positioning and installation of a turbine speed sensor device in a gas turbine engine. It particularly relates to the installation of such a device in a gas turbine engine wherein installation is difficult or not accessible via a straight line from an appropriately located access port on the outer case of the engine.
Gas turbine engines, in common with all engines, require some means for sensing the speed of the output shaft. Such speed sensing is important for several reasons. First, it provides an input to a power output indicator and secondly it assists in control of the engine. For example, a speed sensing device such as envisioned in this invention is capable of detecting a rapid increase in turbine speed resulting from mechanical failure in a gear train which the turbine may be driving. Such a loss of gear teeth in a load can result in explosive termination of engine operation because of the high speeds inherent in gas turbine operations. Consequently, early sensing of a rapid increase in output shaft speed which may be a portent of engine failure is of great importance to prevent destructive turbine overspeeds. Sensing of such an overspeed may be programmed to result in automatic shut off of the fuel supply to the engine.
The design of a gas turbine quite frequently precludes installation of speed sensing devices in the vicinity of the output shaft because of cramped conditions. It should be apparent to those familiar with the gas turbine art that the mechanical method often utilized in internal combustion reciprocating engines wherein a toothed wheel is placed on the drive shaft to drive a tachometer cable which in turn operates a dial type speed indicating device is not acceptable in a gas turbine because of high shaft speeds. Consequently, some other method of speed sensing is necessary. A particularly suitable method includes the positioning of a permanent magnet in the vicinity of a ferromagnetic toothed or rigid wheel or shaft. The permanent magnet carries thereabout a coil with appropriate leads which may be affixed to an electronic device. Rotation of the toothed wheel influences the magnetic field of the permanent magnet thus generating a current in the coil wrapped about the magnet. The current generated in the coil is transmitted to a sensing device which converts voltage fluctuations into a shaft output speed. Such electronic computation is possible because the frequency of voltage fluctuations is directly proportional to the speed of rotation of the shaft. A critical aspect of the use of such magnetic speed sensing device is the positioning of the device at a proper predetermined distance from the toothed or rigid wheel. When access is readily available to the toothed wheel the positioning of such a speed sensing device presents little or no problem. However, in certain gas turbine engines such access is not readily available and therefore the positioning of a small speed sensing device such as is envisioned in this application becomes extremely difficult. It is most advantageous to be able to position the speed sensing device from the exterior of the engine casing and insure that each such installation would locate the speed sensing device at the predetermined distance from a toothed wheel mounted on an output shaft.